1. Field of the Invention
The present invention relates to silicon crystallization, and more particularly, to a method for crystallizing silicon using a laser crystallizing device including a mask, to shorten the process time by decreasing translation of a state on the Y-axis direction of a large-sized substrate.
2. Discussion of the Related Art
As information technologies develop, various displays are in demand. Recently, many efforts have been made to research and develop various flat display panels such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescent display (ELD), a vacuum fluorescent display (VFD), and the like. Some types of the flat display panels have already been applied to various display devices.
LCD devices are widely used because of their characteristics and advantages, including high quality images, light-weight, thin and compact size, and low power consumption so as to be used in place of cathode ray tubes (CRT) for mobile image display devices. An LCD device has also been developed to receive broadcast signals to display, such as a television, a computer monitor, and the like.
Even if there are significant developments in LCD the technology for an image display in various fields, the image quality fails to meet the characteristics and advantages of an LCD display. In order to use a liquid crystal display device as a display device in various applications, development of an LCD device depends on realizing high image qualities, such as high resolution, high brightness, wide screen, and the like, as well as maintaining the characteristics of lightweight, compactness, and low power consumption.
An LCD display device includes an LCD panel displaying an image and a driving unit that applies driving signals to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other so as to secure a space therebetween, and a liquid crystal layer is injected between the first and second glass substrates.
Formed on the first glass substrate (TFT array substrate) are a plurality of gate lines arranged in one direction at fixed intervals a plurality of data lines arranged in a direction substantially perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in pixel areas defined by the gate and data lines crossing each other, and a plurality of thin film transistors switched by signals on the gate lines to transfer signals of the data lines to the pixel electrodes. Formed on the second glass substrate (color filter substrate) are a black matrix layer that shields light in areas outside the pixel areas, an R/G/B color filter layer for realizing colors, and a common electrode for producing an image.
The above-described first and second substrates are separated from each other by spacers to provide a space therebetween, and are bonded to each other through a sealant having a liquid crystal injection inlet. Further, liquid crystal is injected between the two substrates. At this time, the liquid crystal injection method is carried out by maintaining a vacuum state within the gap between the two substrates bonded by the sealant. Then, the liquid crystal injection inlet is dipped in a vessel containing liquid crystal, so as to allow the liquid crystal to be injected into the gap between the two substrates by osmosis. After the liquid crystal is injected as described above, the liquid crystal injection inlet is sealed with an air-tight sealant.
The operating principle of a general liquid crystal display device uses the optical anisotropy and polarization characteristic of liquid crystal. Because of the thin and long structure of liquid crystal, the liquid crystal molecules are aligned to have a specific direction. Also, the direction of the alignment may be controlled by applying an induced electric field to the liquid crystal molecules. Therefore, when the alignment of the liquid crystal molecules is arbitrarily controlled, the alignment of the liquid crystal molecules is eventually altered. Subsequently, due to the optical anisotropy of liquid crystal, light rays are refracted in the direction of the alignment of the liquid crystal molecules, thereby producing image information.
Among current technologies, the active matrix liquid crystal display (LCD), which is formed of a thin film transistor and pixel electrodes aligned in a matrix and connected to the thin film transistor, is considered to be excellent for its high resolution and its ability to represent moving images.
A poly-silicon, having a high electric field mobility and low photocurrent, may be used to form a semiconductor layer of the above-described thin film transistor. The method for fabricating a poly-silicon can be divided into a low temperature fabrication process and a high temperature fabrication process depending upon the fabrication temperature.
The high temperature fabrication process requires a temperature condition approximate to 1000° C., which is equal to or higher than the temperature for modifying substrates. Accordingly, because glass substrates have poor heat-resistance, expensive quartz substrates having excellent heat-resistance should be used. And, when fabricating a poly-silicon thin film by using the high temperature fabrication process, deficient crystallization may occur due to high surface roughness and fine crystal grains, thereby resulting in deficient device application, as compared to the poly-silicon formed by the low temperature fabrication process. Therefore, technologies for crystallizing amorphous silicon, which can be vapor-deposited at a low temperature, to form a poly-silicon are being researched and developed.
The low temperature fabrication process can be categorized into laser crystallizing and metal induced crystallization processes. The laser crystallizing process includes irradiating a pulsed laser beam on a substrate. By using the pulsed laser beam, the solidification and condensation of the substrate can be repeated at a cycle unit of about 10 to 102 nanoseconds. This low temperature fabrication process is known for having the advantage that the damage caused on a lower insulating substrate may be minimized.
The related art silicon crystallization method using laser crystallizing will now be explained in detail. FIG. 1 illustrates a graph showing sizes of amorphous silicon particles by laser energy density. As shown in FIG. 1, the crystallization characteristic of the amorphous silicon may be described as having a first region, a second region, and a third region depending upon the intensity of the laser energy.
The first region is a partial melting region, whereby the intensity of the laser energy irradiated on an amorphous silicon layer melts only the amorphous silicon layer. After the irradiation, the surface of the amorphous silicon layer is partially melted in the first region, whereby small crystal grains are formed on the surface of the amorphous silicon layer after a solidification process.
The second region is a near-to-complete melting region, whereby the intensity of the laser energy, being higher than that of the first region, almost completely melts the amorphous silicon. After the melting, the remaining nuclei are used as seeds for a crystal growth, thereby forming crystal particles with an increased crystal growth as compared to the first region. However, the crystal particles formed in the second region are not uniform. Also, the second region is narrower than the first region.
The third region is a complete melting region, whereby laser energy with increased intensity, as compared to that of the second region, is irradiated to completely melt the amorphous silicon layer. After the complete melting of the amorphous silicon layer, a solidification process is carried out, so as to allow a homogenous nucleation, thereby forming a crystal silicon layer formed of fine and uniform crystal particles.
In the method for fabricating poly-silicon, the number of laser beam irradiations and degree of overlap are controlled in order to form uniform, large and rough crystal particles by using the energy density of the second region. However, the interfaces between a plurality of poly-silicon crystal particles act as obstacles for the electric current flow, thereby decreasing the reliability of the thin film transistor device. In addition, collisions between electrons may occur within the plurality of crystal particles causing damage to the insulating layer due to the collision current and deterioration, thereby resulting in a performance degradation. In order to resolve such problems when using a method for fabricating a poly-silicon by sequential lateral solidification (SLS) crystallization, the crystal growth of the silicon crystal particle occurs at a surface interface between liquid silicon and solid silicon in a direction perpendicular to the surface interface. The SLS crystallization method is disclosed in detail by Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, pp. 956-957, 1997.
In the SLS crystallization method, the amount of laser energy, the irradiation range of the laser beam, and the translation distance are controlled, so as to allow a predetermined length of lateral growth of the silicon crystal particle, thereby crystallizing the amorphous silicon into a single crystal. The irradiation device used in the SLS crystallization method concentrates the laser beam to a small and narrow region, and so the amorphous silicon layer deposited on the substrate cannot be simultaneously changed into polycrystalline. Therefore, in order to change the irradiation position on the substrate, the substrate having the amorphous silicon layer deposited thereon is mounted on a stage. Then, after an irradiation in a predetermined area, the substrate is translated so as to allow an irradiation to be performed on another area, thereby carrying out the irradiation process on the entire surface of the substrate. Alternatively, the laser may be mounted on a stage allowing it to move relative to the substrate.
FIG. 2 is a schematic view illustrating a general SLS (Sequential Lateral Solidification) device. Referring to FIG. 2, the general SLS device includes a laser beam generator 1 for generating a laser beam, a focusing lens 2 for focusing the laser beam generated from the laser beam generator 1, a mask 3 for irradiating the laser beam to a substrate 10, and a reduction lens 4 for reducing the laser beam passing through the mask 3 to a smaller size. The laser beam generator 1 generally generates a light beam with a wavelength of about 308 nanometers (nm) using XeCl or 248 nanometers (nm) using KrF in an excimer laser. The laser beam generator 1 discharges an untreated laser beam. The discharged laser beam passes through an attenuator (not shown), in which the energy intensity is controlled. The laser beam is then irradiated to the focusing lens 2.
The substrate 10 having an amorphous silicon layer deposited thereon is fixed to an X-Y stage 5, which faces the mask 3. In order to crystallize an entire surface of the substrate 10, the X-Y stage is minutely displaced, thereby gradually expanding the crystallizing region. The mask 3 includes an open part ‘A’ allowing the laser beam to pass through, and a closed part ‘B’ absorbing the laser beam. The width of the open part ‘A’ determines the lateral growth length of the grains formed after the first exposure.
A method for crystallizing silicon using the general SLS device of FIG. 2 according to the related art will be described as follows. FIG. 3 is a cross-sectional view schematically illustrating the laser crystallizing process according to the related art. As shown in FIG. 3, a buffer layer 21 and an amorphous silicon layer 22 are sequentially formed on a substrate 20. Then, a mask (not shown) with sequentially alternating open parts and closed parts is placed over the substrate 20 having the amorphous silicon layer 22 deposited thereon. Thereafter, the laser irradiation process is performed on the amorphous silicon layer 22. The amorphous silicon layer 22 is generally deposited on the substrate 20 by a chemical vapor deposition (CVD) method, which results in a high content of hydrogen within the amorphous silicon layer 22 immediately after the deposition. However, when heated, hydrogen tends to escape from the thin film. Therefore, a primary heat treatment may be carried out on the amorphous silicon layer 22 in order to perform the dehydrogenation process. When the hydrogen is not eliminated during a prior process, the surface of the crystallized silicon layer becomes rough, thereby resulting in the poor electrical characteristics.
FIG. 4 is a plane view illustrating the substrate including the amorphous silicon layer having a crystallized region with the dehydrogenation process. As shown in FIG. 4, because the laser beam width and the size of a mask are limited, it is impossible to crystallize the entire surface of the substrate 20 at once with a single shot of the laser beam. Accordingly, as the size of the substrate increases, it is necessary to align a single mask several times, and to perform the crystallization process repetitively. If a crystallized region with a size C is defined as one block, the laser beam is irradiated several times to crystallize one block.
FIG. 5 illustrates the crystallized region of the amorphous silicon layer corresponding to the open part after the first irradiation of laser beam with the SLS device of FIG. 2. As shown in FIG. 5, a single shot of the laser beam is irradiated through the mask (not shown) placed over the amorphous silicon layer 22. At this point, the irradiated laser beam passes through the plurality of open parts ‘A’ formed on the mask. The laser beam then melts and liquidizes the amorphous silicon layer 22. The intensity of the laser energy is selected in the complete melting region, whereby the silicon layer is completely melted.
Similarly, after the laser beam irradiation, a silicon grain 33 grows laterally from the interfaces 32 between the amorphous silicon region and the completely melted and liquidized silicon region towards the center of the irradiated area. The lateral growth of the grain 33 proceeds in a horizontal direction perpendicular to the interface 32. In the irradiated area corresponding to the open part of the amorphous silicon layer 22, when the width of the open part ‘A’ of the mask is half or less than half the growth length of the silicon grain, the grain growing inwards on both sides of the silicon region in a perpendicular direction comes into contact at a mid-point 31, thereby causing the growth to stop. As described above, after the crystallizing process of a first irradiation, the number of crystallized regions formed in one block is the same as the number of open parts ‘A’ formed on the mask (‘3’ of FIG. 2).
Subsequently, in order to achieve further growth of the silicon grain, the stage having the substrate is translated to carry out another irradiation process on an area adjacent to the irradiated area of the prior process. Thus, another crystal is formed, the new crystal being connected to the crystal formed after the first exposure. Similarly, crystals are immediately formed on each side of the completely solidified regions. Generally, the crystal growth length processed by the laser irradiation process and connected to the irradiation part is in the range of 1.5 to 2 micrometers (μm). By repetition of the aforementioned process, as shown in FIG. 4, the amorphous silicon layer corresponding to one block C is crystallized. However, because the width of the open part A of the mask is smaller than that of the closed part B, the mask is translated several times to crystallize one block C. Accordingly, the process time for crystallization of the amorphous silicon layer increases with each translation of the mask or the stage, thereby lowering yield.
A related art mask having open and closed parts of the same width will be described as follows.
FIG. 6 is a plan view illustrating another mask used in the related art SLS device. Referring to FIG. 6, a mask 40 of the related art SLS device includes open and closed parts A and B formed alternately in a horizontal direction. It is possible to define respective widths ‘p’ and ‘q’ of the open and closed parts A and B. In FIG. 6, the open part A has the same width as that of the closed part B. Using this mask, it is possible to crystallize one block of the substrate through a first and second exposure process. Crystallization blocks are formed on the substrate by the primary laser beam passing through the plurality of open parts A of the mask 40. In this case, one block corresponding to the plurality of open parts A and the closed parts B in-between has a length ‘S’ and a width ‘L’ corresponding to the length of one open part A.
FIG. 7A to FIG. 7C are plan views illustrating a related art silicon crystallization method using the mask of FIG. 6. Generally, a laser beam irradiation device has a reduction lens below the mask 40, whereby a laser beam pattern irradiated on the substrate 50 through the mask 40 is reduced by a constant rate of the reduction lens. For example, in case when the reduction rate is ‘5’, the length and width of the open part A of the mask 40 are set as ‘L’ and ‘p’, one crystallization region of one block irradiated with the laser beam through the open part A of the mask has a length of L/5 and a width of p/5. Herein, as one laser pulse is irradiated to the substrate 50 through the mask 40 and the reduction lens, the crystallization process is carried out on a block having a width ‘l’ and a length ‘s’.
As shown in FIG. 7A, the substrate 50 is on the stage (not shown), and then the mask 40 is arranged corresponding to the substrate 50. Then, as shown in FIG. 7B, the substrate 50 corresponding to the mask 40 is translated along the X-axis direction to the right a distance corresponding to the length ‘l’ of one crystallization region, thereby producing lines of crystallization corresponding to the number of the open parts A of the mask 40. Accordingly, the crystallization for the X-axis direction to the right is completed as the open parts A reach the edge of the substrate 50.
As shown in FIG. 7C, the stage is translated upward along the Y-axis direction at a range corresponding to a width ‘a’ of one crystallization region, so that open parts A of the mask 40 correspond to the non-amorphous regions. Then, the stage is translated along the (−) X-axis direction at a distance corresponding to the length of one crystallization block, whereby the crystallization proceeds along the line of the open parts A of the mask by translating the substrate 50 relative to the mask 40. After completing the crystallization along the (−) X-axis direction on the substrate 50, the crystallization area on the substrate 50 is the length of the substrate×the length (s+a), which includes the length ‘s’ of one block and the width ‘a’ of one crystallization region. Subsequently, when the stage is translated upward along the Y-axis direction a distance corresponding to the length ‘s’ of one block, the process of FIG. 7A to FIG. 7C is repetitively carried out on the substrate 50, thereby completing the crystallization on the entire surface of the substrate 50. In the related art silicon crystallization method, the crystallization is processed along the X-axis direction. However, it is possible to process the crystallization on the Y-axis direction by rotating the mask 40 90°.
However, the related art silicon crystallization method has the following disadvantages. In the related art silicon crystallization method, after crystallizing the substrate along the X-axis direction to the right or the X-axis direction to the left, when the laser irradiation crystallizes the uncrystallized portions by changing the translation direction of the stage along the X-axis, it requires the minute translation of the stage in the Y-axis direction. Generally, the stage is translated at a range smaller than the width of one crystallization region, for example, between 0.1 μm and 9.9 μm. In this case, the stage along the direction of the X-axis is stopped for a predetermined time period to translate the stage in the direction of the Y-axis. It requires the time t1 to stop the stage from a predetermined speed, the stage stop time is t2, and the acceleration time t3 is the time it takes to translate the stage in the X-axis direction at the predetermined speed. Accordingly, the time (t1+t2+t3) required for changing the translation direction of the stage increases, thereby increasing probability of errors. Especially, in case of the large-sized substrate, this problem becomes more serious.